A Federal Court injunction holding up a $1.25 billion highway in Western Australia has been extended. 

Bulldozers had just begun clearing bushland for the final leg of the contentious Bunbury Outer Ring Road project last week when work suddenly stopped.

A local environmental group had mounted a legal challenge, and an injunction was enforced after just two days’ work on the project.

In court, Justice Craig Colvin has heard arguments for and against Environment Minister Tanya Plibersek's approval of the project in late June.

Justice Colvin ordered a pause on construction while the case is under way.

Lawyer Angel Aleksov, representing the Friends of the Gelorup Corridor group, is arguing that the ministerial approval of the project was unlawful, as much of the environmental mitigation plan that the project's approval relies on will not occur until much later.

Mr Aleksov said it would be too late for animals such as the critically endangered western ringtail possum, which “might vanish from this earth”.

Mr Aleksov claims Main Roads WA assumed that 9,000 western ringtail possums remained in the region, but Wikipedia allegedly showed there could be as few as 3,000 possums left.

While he conceded that delaying the project is costly, he argued that the cost “does not outweigh risk to species that might leave this earth”.

The project was delayed from April to August, at a cost of up to $10 million.

“There are large financial ramifications, but there are very, very large environmental consequences,” Mr Aleksov said.

The Solicitor General of WA, Joshua Thomson, and Commonwealth lawyer Emrys Nekvapil all argued that relevant procedures had been followed to mitigate environmental damage.

Mr Thomson warned that if the project was stalled, it would have to be delayed another seven months to allow for the western ringtail possums' breeding patterns.

The case continues.